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We have not been able to obtain accurate measurements of all parts of this instrument. The telescope is 83 feet long, with a clear aperture of 7 inches. It is supported in the middle of the axis, between two piers; it has two 4 feet circles, one on each end of the axis, divided on gold into arcs of 2′ value. Each circle is provided with six reading microscopes. The telescope has two micrometers, one moving in azimuth, the other in altitude. It is so contrived that the wires, and not the field, are illuminated; and every eye-piece, even of the highest powers, just as it is used, and without alteration of any kind, becomes a collimating eye-piece, by simply turning the telescope down upon a basin of mercury. At one end of the axis, and concentric with it, are two cross lines situated in the focus of a lens placed at the other end of the axis. Upon the prolongation of this axial line at either end, and attached to the same piers which support the telescope, are placed two collimators, which are ingeniously contrived to stand in the place both of the striding level and meridian marks, and which are to be used for determining the level of the instrument, the figure of the axis, and the eccentricity of the pivots. There is a further provision for detecting the slightest changes in the deviation of the level of the axis from a normal state, on account of unequal expansion of the supporting piers, which are of granite. This consists of another beautiful contrivance by means of a couple of invariable rods, or rods of well-determined expansibility, which are inserted vertically through the piers; and which, by turning a mirror, show the minutest alterations which can take place from this source, in the level or inclination of the axis of the instrument. Thus the imperfections of the spirit level are in a great measure avoided; and all the advantages, with but a part of the inconveniences, of a striding level to reverse with the instrument, are secured.

Appreciating the advantages which these fine instruments afford, the superintendent appears to have begun with the determination of so employing them, that their results, while they should be the most useful, would at the same time most redound to the credit of the observatory and the navy, and to the honor of the country. The national observatory being a naval institution, its first object and duty would seem to be to undertake those observations which are most immediately connected with navigation, and the results of which might afford the data requisite for calculating an American Nautical Almanac. Accordingly the naval astronomer at once com. menced with a regular and systematic series of observations upon the sun and moon, the planets, and a list of fundamental stars, comprising those of the greatest magnitudes and of the most favorable positions, to be used as the standard stars in the almanac. But these did not give full employment either to the instruments, the observers, or the computers. Accordingly, after having adjusted his plans, Lieut. Maury took an early opportunity, in 1845, of making them known to the Secretary of the Navy, and of obtaining from him authority to undertake observations for a most extensive

catalogue of stars. This work, when completed according to the original design, will be one of the greatest contributions to the science of astronomy that has ever been made by a single observatory; for it contemplates nothing short of a regular and systematic examination, with some one or other of these excellent instruments, of every point of space in the heavens that is visible to us, and of assigning position, color, and magnitude to every star that they are capable of reaching. This will be the work of years. Lieut. Maury's plan and arrangements differ in many of their details from those adopted by any other astronomer who has engaged in similar undertakings; and as they are calculated to afford more satisfactory results than those which have gone before, a brief sketch of them may prove of interest to the friends and patrons of American science.

In Bessel's Zones, the most extensive catalogue at present in existence, the plan was to sweep up and down a zone in the heavens, of a certain breadth, and to have two persons engaged at the same time; one as an assistant to read the instrument, the other to observe the star. By such a plan the declination of each star observed depended upon one reading of a single microscope. By the Washington plan, the services of an assistant to read the instrument are dispensed with, and the declination of every star depends upon the readings of six microscopes instead of one. This improvement has been accomplished by an ingenious contrivance, which gives motion to the eye-piece instead of the telescope during the sweep over any particular zone or belt. The micrometer diaphragm for cataloguing is provided with a number of parallel wires, the intervals of which have been carefully determined; and, by giving the eye-piece a motion in altitude as well as in azimuth, each instrument is made to cover a belt of from 40 to 50' broad in declination. The Nadir point being carefully observed, the telescope (that of the mural, for instance), being set for the belt to be swept, and all the microscopes carefully read and recorded, the observer takes his position at the eye-piece, and begins to move it up and down, the instrument itself remaining fixed as he reads it. In whatever part of the field a star appears, a micrometer wire is close at hand (so that there is no loss of time, as with a single wire, in running the micrometer wire from one edge of the field to the other), and the star is bisected with this nearest micrometer wire, while the time at which it transits the several vertical wires is also noted. The number of the bisecting wire and the reading of the micrometer being now entered, the observation is complete.

The observer thus keeps his eye at the telescope for hours at a time; and under favorable circumstances can observe with ease two or three hundred stars during the night. The meridian circle, in the same way, will occupy the belt below this; while the transit instrument, which the ingenuity of the astronomer has converted into a difference of declination instrument, occupies the belt above, each instrument overlapping the belt of the other by 4' or 5'; the stars, in the parts thus overlapped, being common to two in

struments. Thus, the meridian circle and transit instrument establish the right ascension of certain stars which are used by the mural as so many points of departure; while the mural or meridian circle establishes the declinations of certain stars which serve the transit instrument as points of departure for difference of declination. This is the first sweep. The next night, the instruments change places, and go over the same ground; i. e. the meridian circle covers the same belt to-night which on the former night was swept by the mural. Supposing the two nights equally favorable, and the instruments of equal power,—the latter of which is actually the case, the former seldom,—all the stars that were seen in the first sweep, by the mural, should be observed in the second by the meridian circle. The two lists are immediately compared, and should there be any discrepancies between them, the large equatorial is put in pursuit of the peccant stars.

This great American work contemplates the examination of every star down to the tenth magnitude, not only in the zodiac, but in the whole heavens; it includes a revision of all the European catalogues; and, while it looks to the discovery of new planets and unknown stars, it also aims to detect the disappearance of any stars known to existing catalogues. In short, the Washington catalogue aims at that degree of perfection, which, when future astronomers shall find an unknown star, may justify the remark this star is new in the heavens, because it is not in the Washington catalogue." Lieut. Maury's original plan contemplated also measurements for angle of position and distance of all double or multiple stars, together with accurate drawings of all clusters and nebulæ. This was to be the work of the equatorial; but the undertaking to glean after the meridianal instruments gives this telescope full employment for the present.

The observatory commenced its operations in 1845. The first volume, a quarto of five hundred pages, has already been published, and has elicited many and high commendations, both at home and abroad. In the opinion of one of the most distinguished astronomers of Europe, it has, at once, placed our national observatory in the front rank with the oldest and best institutions of the kind in Europe. The volume for 1846 is in press. The catalogue for that year will number some 12 or 15,000 stars, most of them unknown to any existing catalogues; the whole work will comprise a quarto volume of not less than 1,000 pages, and will be the largest work of the kind ever published by any observatory as the result of a single year's

labor.

THE GREAT TELESCOPE AT CAMBRIDGE, MASS.

Some Account of the Great Telescope recently erected at the Observatory at Harvard College, Cambridge, Muss. By W. Cranch Bond, A. M., Director of the Observatory.

The history of this "Grand Refractor," as it is named by its makers, is briefly as follows:- Early in the month of March, 1843, a very remarkable appearance presented itself in the heavens. A comet of unusual magnitude and brilliancy was attracting the attention of the public, and the observatory at Cambridge was looked to for information respecting the movements, &c., of this mysterious stranger. In answering this requisition, the opportunity was embraced to make known the want which existed in the apparatus of the observatory, of instruments adapted to extra-meridional observations. The appeal then made was promptly responded to, and the deficiency supplied. To the untiring and efficient exertions of the venerable ex-president of the university, the Hon. Josiah Quincy, may be attributed, in a very great degree, the success of this undertaking.

On an afternoon early in the month of March, 1843, an informal meeting of three or four individuals interested in the subject was held at the office of the American Insurance Company in Boston. The proceedings of this meeting were cordially seconded by the American Academy of Arts and Sciences, and in consequence a regular meeting of merchants and other citizens of Boston was held at the hall of the Marine Society, to consider the expediency of procuring a telescope of the first class, for astronomical observation. At this meeting, the question was decided in the affirmative, and a subscription, to the amount of twenty thousand dollars, recommended, to defray the expense. This amount was immediately furnished, - an individual not less eminent for his philanthropy than for his love of science taking the lead by a donation of five thousand dollars, independent of the above-mentioned sum, to be appropriated to the building of an observatory tower.

After a general correspondence with the best informed astronomers and opticians in Europe, it was decided to employ Messrs. Merz and Mahler, of Munich, in Bavaria, to make the instrument. These gentlemen are the successors of the celebrated Frauenhofer, and the proprietors of the far-famed Optical Institute of that city. They bound themselves by contract to complete two object glasses, of the clear aperture of fifteen inches, to be at least equal to that furnished to the noble instrument now mounted at the Imperial Observatory of Russia, at Pulkova. On being notified of the completion of these object glasses, the agent of the university, Mr. Joseph Cranch, of London, accompanied by Mr. William Simms, an eminent maker of astronomical instruments, proceeded to Munich, and, after careful trial and examination, made the required selection. The selected object glass was

received at this observatory on the 4th of December, 1846; the great tube and its parallactic mounting did not reach us until the afternoon of the 11th of June, 1847. It then required nine days to prepare the stone pier to which the telescope was to be attached. It being necessary that the upper surface of the stone should have an exact inclination in order to secure for the hour axis of the instrument an angle of elevation equal to the latitude of the place, a triangular frame of wood was made, having one of its sides vertical and another horizontal, the former watched by a plumb line and the latter by a spirit level, for the guidance of the workmen; this was found to answer the purpose perfectly, it being easily set whenever a trial of the inclination was thought necessary.

A strong platform was raised round the pier, with apparatus for raising the weighty machinery into place; the boxes containing the different parts of the telescope, which altogether were estimated to weigh about four tons, were arranged on the floor; and the process of putting together was commenced on the morning of the 23d of June. No accident occurred, and no step had to be retraced during the work. On the evening of the next day, the telescope was directed upon celestial objects.

It may here be proper to give a description of the building, with its revolving dome and pier, which had been prepared for the reception of this "Grand Refractor."

The part of the observatory which has been appropriated to this instrument is a square tower of thirty-two feet on a side. The walls are of brick, resting on a granite foundation. The interior is gradually brought into a circular form of thirty-one feet diameter, surmounted by a granite circle, on which is laid an iron rail of ten inches width, hollowed in the middle to serve as a track for the eight-inch iron balls on which the dome rolls. The dome is thirty feet, interior diameter, with an opening five feet wide, extending beyond the zenith. The shutters to this opening are raised and closed by means of endless chains working in teethed pulleys, and are easily managed by a winch and pinions, geered into wheels of one foot diameter. They are perfectly weather proof. To the lower edge of the dome is affixed a grooved jron rail, similar to the one laid on the granite cap of the walls. Eight iron balls, which had been smoothly and truly turned, were placed at equal distances round the circle, and the dome gradually let down to rest upon them. Although this dome is estimated to weigh about fourteen tons, yet it can be turned through a whole revolution by a single individual, without any very great exertion, in thirty-five seconds.

The central pier, for the support of the telescope, is of granite, and is in form the frustum of a cone, of twenty feet diameter at the base, and ten feet at the top. It is forty feet high, and rests on a wide foundation of grouting, composed of hydraulic cement and coarse gravel, and is entirely detached from every other part of the building. Upon the top of the pier is laid a circular cap-stone, ten feet in diameter and two feet thick; on this

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